I. Field of the Invention
The invention relates generally to wireless communications. More particularly, the invention relates to multiple access in a wireless communication system.
II. Description of the Related Art
In a typical wireless communication system, a plurality of mobile stations communicate through a common base station. Because the base station has finite resources available, the mobile stations compete for access to the base station resources. FIG. 1 is a block diagram showing a typical modern wireless communication system 10. The system is comprised of a series of base stations 14. A set of mobile stations 12 communicate with the base stations 14. The mobile stations 12 communicate with the base stations 14 over a forward link channel 18 and a reverse link channel 20. As used herein, the term “channel” refers to both a single communication link between the base station and a specific mobile station as well as a grouping of communication links, typically having a common function. FIG. 1 shows a variety of types of mobile stations. For example, FIG. 1 shows a hand-held portable telephone, a vehicle mounted mobile telephone and a fixed location wireless local loop telephone. Such systems offer voice and data services. Other modern communication systems operate over wireless satellite links rather then through terrestrial base stations.
An industry standard for a wireless system using code division multiple access (CDMA) is set forth in the TIA/EIA Interim Standard entitled “Mobile Station—Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System”, TIA/EIA/IS-95, and its progeny (collectively referred to here in as IS-95), the contents of which are also incorporated herein by reference. Among other channels, IS-95 defines a reverse link random access channel which is used by the mobile stations to communicate with a base station. The access channel is used for short signaling message exchanges such as call originations, responses to pages and registrations. For example, for prolonged bi-directional communications, a dedicated forward link and reverse link traffic channel pair are established between the mobile station and the base station. The access channel can be used to transfer information from the mobile station to the base station before the traffic channel is established in order to facilitate establishment.
The access channel defined by IS-95 is a random access channel meaning that a mobile station randomly chooses a portion of the access channel resources over which to transmit an access probe. Due to the random nature of the access channel, there is no guarantee that only a single mobile station will attempt access on the chosen portion. Therefore, when an access probe is sent, it may fail to be received by the base station for one of several reasons. It may fail because the power level received at the base station is too low compared to the current interference levels. It may fail because another mobile station attempts to use the same portion of the access channel resources at the same time causing a collision. In any case, when the access probe is not received at the base station, the mobile station randomly selects another portion of the access channel resources and attempts access to the system, perhaps using a higher signal level. In order to avoid a series of lockstep failures between two mobile stations after an initial collision, the retransmission process is also randomized.
In order to select a portion of the access channel resources, according to IS-95, the mobile station randomly selects one of a set of one or more access channels defined by CDMA techniques. Once an access channel is selected, the mobile station is constrained to begin transmission of the access probe at one of a set of re-occurring slot boundaries. The mobile station randomly selects a slot boundary and begins transmission. Such operation is referred to as slotted aloha operation and is well known in the art.
One key aspect of a random access system is load control. Load control is used to statistically control the rate at which access probes are received at the base station. Load control in a slotted aloha system is important because as the number of access attempts increases, the number of collisions also increases. As the loading further increases, the number of successful access attempts actually begins to fall due to the system resources being consumed with collisions. Therefore, in a slotted aloha system, it is advantageous to keep system loading at less than 18% of the fully loaded capacity, otherwise unstable behavior can result.
Loading is also a function of the amount of interference in the system. The available capacity of a system decreases as the interference increases. As the load on the random access channel increases, it may cause significant interference to other channels in the system such as the traffic channels. According to IS-95, loading on the access channel is controlled by the insertion of random delay (called access probe back-off) between a failed access attempt and a follow up attempt. However, IS-95 lacks any mechanism for quickly enabling and disabling access to the access channel in order to control loading.
According to IS-95, when a mobile station sends an access probe, it transmits a uniquely identifying number such as the electronic serial number (ESN) of the mobile station along with other information in a preamble. In addition, the access probe comprises a message which specifies the purpose of the probe or carries user data. For example, the message may designate a telephone number for use in a call origination. An access probe is typically between 80 and 150 milliseconds (msec) in duration.
According to IS-95, the mobile station initially transmits the access probe at a first level. If the base station does not respond with an acknowledgment after a predetermined amount of time, the mobile station continues to repeat the access probe at increasingly higher power levels.
This method of access does not yield a very efficient use of system resources. First, the access probe is fairly lengthy and the mobile station continues to transmit the entire access probe even if the base station is unable to receive the access probe, thus, spewing un-useful energy into the system, wastefully expending mobile station resources and reducing system capacity. According to IS-95, once the mobile station has begun to transmit, no power control mechanism exists by which the base station can increase or decrease the transmit power. If the reverse link is subjected to a deep fade, the transmission may fail and the mobile station retransmits the message at a higher power level which may not be necessary in the absence of the fade. The base station has no means to request more power during the deep fade nor to request a reduction in power during the subsequent retransmission. In addition to consuming significant system resources, the access method according to IS-95 can stretch to cover a significant amount of time adding delay to the system. According to IS-95, data is transmitted over the access channel at only one data rate regardless of the amount of data or the quality of the connection between the mobile station and the base station.
Thus, there has been a need in the art to develop a multiple access system which introduces less delay and makes more efficient use of the available system resources.